Is second from lowermost member of Hornbrook Formation. Named for outcrops in Osburger Gulch east of Klamath River about 3.5 km south of Hornbrook. Type section designated near mouth and along north flank of Osburger Gulch (SW/4 sec.33 T47N R6W, Hornbrook 15' quad, Siskiyou Co, CA). Consists dominantly of gray sandstone with some conglomerate, siltstone, and shale. Is from 75 to 150 thick (116 m thick at type). Locally absent near Ashland but may be 250 ft thick at north end of Cottonwood Valley. Conformably overlies Klamath River Conglomerate Member (new) or where member is absent unconformably overlies pre-Cretaceous basement rocks. Conformably underlies Ditch Creek Siltstone Member (new). Abundant molluscan fossils indicate Cenomanian age in parts of Jacksonville and Dark Hollow areas, a Turonian age in Bear Creek Valley, Cottonwood Creek Valley, and Shasta Valley areas, and locally an early Coniacian age in Hornbrook area.

Source: GNU records (USGS DDS-6; Menlo GNULEX).

Measured type section, detailed lithology, thickness, depositional environment, lists of fossils included in study. Age of unit adopted herein is, excluding Grave Creek area, late Albian(?) to early Coniacian based on ages of megafossils (Sliter and others, 1984).

Source: GNU records (USGS DDS-6; Menlo GNULEX).